Class LayerWidget

The layer widget represents a transparent widget which functionality is similar to JGlassPane.
The layer widget is used for speed optimalization too since it is not repainted when the widget is re-layout.

It can be used widgets organization. A scene usually has layer widgets directly underneath.
E.g. each layer widget is used different purpose:
background for widgets on background,
main layer for node widgets,
connection layer for edge widgets,
interraction layer for temporary widgets created/used by actions.